Prime minister Theresa May speaks during a debate on her Brexit “plan B” in Parliament.

Senior ministers are arguing that the controversial Irish backstop issue that stands in the way of a Brexit deal can be solved if the UK commits to a form of permanent customs union with the EU, BuzzFeed News has learned.

Sources across Whitehall are increasingly convinced they will be able to secure a political win by adding either an end date or an exit mechanism to the backstop — the insurance policy to prevent a hard border — that the House of Commons says has to change for a Brexit deal to pass.

One proposal being suggested by ministers and MPs is for the UK to make a commitment in the political declaration outlining the future UK–EU relationship to join a version of a permanent customs union. This commitment could “neutralise” the backstop and see the EU give way on an end date or exit clause, the MPs believe.

Staying in a customs union would remove the need for customs-related checks between the UK and the EU, including at the Irish border — a crucial element of the backstop.

“It could be possible to change the backstop in exchange for some sort of permanent customs union or arrangement in the political declaration,” said a government source, stressing this was just one option being looked at. “It would not be a fig leaf, it would be a fig tree.”

Theresa May has so far insisted that the UK will leave the customs union, as well as the single market, in order to strike ambitious trade deals around the world and end the free movement of people. In her Lancaster House speech of January 2017, May said she had an “open mind” about a customs agreement with the EU.

Brussels has been firm in making clear that the prime minister is the one who needs to compromise on her red lines if she wants to change the dynamics of the overall deal.

Francois Lenoir / Reuters

EU chief Brexit negotiator Michel Barnier.

European Commission president Jean-Claude Juncker has reportedly told May that a permanent customs union would be the price for revising the backstop.

Irish Taoiseach Leo Varadkar said earlier this month that if the UK decided to stay in a customs union, then “in those circumstances I think the European Union would evolve its position as well”. “If the UK were to turn around and say that they have changed their mind on a customs union, we wouldn’t say sorry it’s too late,” he added.

Last week, the EU’s chief Brexit negotiator, Michel Barnier, echoed those comments, telling RTÉ that Brussels was willing to work with the UK on a “more ambitious” political declaration.

A UK commitment to a customs union has reportedly previously been backed by Number 10 chief of staff Gavin Barwell, but it would cause major political problems for the Conservatives. Most Tory Brexiteers oppose the idea because it would restrict the ability to strike trade deals with other countries.

“It will not fly with the ERG,” a senior Brexiteer told BuzzFeed News, referring to the European Research Group of hardline Tory Eurosceptics.

But senior Tories on both the Leave and Remain sides told BuzzFeed News that there was a way the plan could win the support of the Democratic Unionist Party, some Brexiteers, and the Labour Party, whose Brexit policy is based on joining a customs union with Brussels.

DUP sources have previously indicated they could support a deal that keeps the whole of the UK in a customs union with the EU if it removed the threat of the backstop.

Two prominent Brexiteers told BuzzFeed News a commitment to some form of customs union in the political declaration would be preferable to the backstop.

“We cannot leave the backstop. A permanent customs union you could leave,” said one, referring to the fact any commitment to a customs union would come in the non–legally binding political declaration, rather than the legally binding withdrawal agreement.

A second agreed, arguing that a future prime minister could remove the UK from its commitment to a customs union and seek a looser free trade deal: “With the political declaration it doesn’t have the same legal weight. A different person in charge could get us out.”

One of the Brexiteers said that if the proposal was sold as a “single customs territory” — language already in the political declaration — then it could win the support of some Brexiteer MPs. “There’s a route through there,” they said.

The other Brexiteer said: “All the talk has been about the backstop. We voted last night that we’d back a deal if the backstop was solved. If they said to us, ‘we did what you asked, we changed the backstop, now meet us halfway’, we would face a conundrum.”

A commitment to a permanent customs union would also place pressure on Jeremy Corbyn to back a deal. At Prime Minister’s Questions on Wednesday, the Labour leader made clear this was his main aspiration, telling the Commons: “Any solution must involve a customs union.”

On Wednesday afternoon, May and Corbyn met in her Commons office where they held what was described as a “detailed” discussion about a customs union.

A Labour spokesperson said: “The government has its own set of priorities and red lines, but Jeremy and Theresa May had a serious engagement including on the detail of a customs union.”